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STATISTICAL ESTIMATES OF VARIANCE FOR 15N ISOTOPE DILUTION MEASUREMENTS OF GROSS RATES OF NITROGEN CYCLE PROCESSES
Citation:
Myrold, D. D., J. B. Cliff, AND D L. Phillips. STATISTICAL ESTIMATES OF VARIANCE FOR 15N ISOTOPE DILUTION MEASUREMENTS OF GROSS RATES OF NITROGEN CYCLE PROCESSES. Presented at 4th International Conference on Applications of Stable Isotope Techniques to Ecological Studies, Wellington, New Zealand, April 19-23, 2004.
Description:
It has been fifty years since Kirkham and Bartholmew (1954) presented the conceptual framework and derived the mathematical equations that formed the basis of the now commonly employed method of 15N isotope dilution. Although many advances in methodology and analysis have been made in the intervening years, researchers still face several questions in calculating gross rates. We provide guidelines for the choice and proper use of the different gross rate equations and a means of estimating the variance associated with using these equations.
15N isotope dilution experiments require the destructive sampling of soils at two, or more, time points. The requirement for destructive sampling means that paired (i.e., before and after) samples are not available. The mathematical equations for calculating gross rates must, therefore, use mean values of pool sizes and isotopic abundances for the two time points. These means, of course, have variances associated with them, but this information is typically not utilized, limiting the ability to make statistical comparisons of gross rates. We provide a comparison of three methods for estimating the variances of gross rates using modeled data: (i) all possible pairs, (ii) bootstrapping, and (iii) error propagation. Our results show that all approaches give similar estimates of the variances of gross rates. An advantage of the error propagation method, however, is that simple algebraic formulae can be used to calculate the variance.